Steel recuperator



y 13, 1953 G. SCHI- ZFELS ET AL 2,834,581

STEEL RECUPERATOR I Filed May 18, 1953 INVENTORS GERHARD Scum-'54s Z Geno WEZLENS/CK y HUGO TREBBE ATTORNEYS.

United States Patent '0 STEEL RECUPERATOR Gerhard Schefels, Gerd Wellensiek, and Hugo Trehbe, Wilhelm-Marx-Haus, Dusseldorf, Germany Application May 18, 1953, Serial No. 355,468

Claims priority, application Germany May 20, 1952 4 Claims. (Cl. 257--224) In the furnace industry recuperators are required for waste gases in which the gases are introduced into the recuperator at temperatures of up to 1100 C. The invention provides a way of using steel recuperators for high temperatures instead of the recuperators consisting of firebricks, hitherto used.

Unless special measures are taken at the high temperatures existing, the heat-exchanging steel surfaces are heated to a white heat. The use of the known constructions of steel recuperator with heat-resistant steel tubes or other metal elements, in which the heating gas flows around the tubes, involves very great difiiculties, particularly when the hot gases contain a large proportion of dust.

Thus, for example, in large apparatus, it is practically impossible to remove all the dust deposited in the tube system, so that at the less accessible positions pockets are formed which form an increasing amount of sinter.

In these cases it is better for the dust-containing gas to flow through straight and wide tubes without any reversal. In such an arrangement the tendency of the dust to deposit is much less than in transverse flows of any type, and the deposited dust may be removed reliably and conveniently.

In Figure l a recuperator is illustrated, in which the dust-containing gas flows through vertical tubes.

a denotes the feed conduit for the waste gas, or, in directly heated recuperators the furnace chamber, from which the gas, such as the waste gas of a furnace, flows through the pipes b-only one of which is shown for the sake of clarity-into the upper collecting space above a plate In leaving the recuperator through the outlet pipe (1. While this construction in itself has been found satisfactory, it cannot be used for the extremely high temperatures in the hitherto known form. Constructions of this type have shown that in certain cases the tubes b are liable suddenly to tear oil. It was eventually found that the reason was that with dust-containing gases some tubes become more dust-laden than others and, under some circumstances, may become completely blocked, and no heat-dissipating gas will flow through the tubes, the heatabsorbing cooling gas however flowing around them, as before. The dust-blocked tubes, will therefore, become considerably cooler than the others, will contract accordingly, and tear otf.

Attempts have been made to overcome this disadvantage by providing a movable bearing 1 for the tubes, but there is then the problem of sealing the movable bearing I. Thus, for example, stutling-boxes of known construction with asbestos packing have been used which are provided at the cooler and movably mounted ends of the pipes.

When these stufiing boxes were fully tightened so that they were completely sealed, the friction on thewalls of the boxes was so great upon moving the tubes, that the walls of the tubes, the strength of which at the hot end was considerably reduced at high temperatures, could not withstand the pressure, and were deformed.

Tests have shown that these stufiing-boxes presented 2,834,581 Patented May 13, 1958 frictional forces of up to 500 kilograms and more per tube. Moreover, stuffing-box packings of this type do not remain tight, because the tubes, when heated, expand radially and force the packing aside. When the temperature is reduced, the tubes will contract again, the

packing does not follow, and a gap is formed and there is leakage.

In recuperators adapted for use with high temperatures, the sealing and movable bearing 1 does not form the only problem. As shown in Figure 1, if the tubes b are merely welded into the lower tube base plate e, difficulties will be experienced in the chamber a at high temperatures, as, even when cooled by the air, the plate assumes such high temperatures that it softens and is progressively deformed. Thus, the radiation in the chamber a which, for example, assumes a temperature of 1100 C. is so high that it exceeds the heat transmission by convection by approximately five times its value. According to the laws of heat transmission, it follows therefore that the temperature of the tube base plate e approaches the temperature of the furnace chamber a. At such temperatures etlective operation of the tube base plate is problematical.

According to the invention, means are provided for reducing the temperature in all the wallsof the recuperator. These means also render it possible to use -(instead of the expensive austenitic chrome nickel steel tubes), chrome silicon steel tubes which are just as heatresistant, but not too heat-retaining, and which are considerably less expensive. The means consist in that, by adequate determination of the cross-section at which the air is conducted around the tubes, a coefiicient of heattransmission is obtained on the air side, which, at the hot as well as at the cold ends of the'tubes, exceeds the mean coelficient of heat transmission of the air, and the mean value of which exceeds the mean value of the coeflicient of heat transmission on the gas side. -It is known that the coetficient of heat transmission has the dimension kilogram-calorie/square metre per hour C. In other words, the air velocity is increased both at the hot and at the cold' ends, and the coeflicient of heat transmission of the air in the recuperator is kept higher than that of the gas. In this way, the wall temperatures of the tubes and base plates are considerably reduced, and it is also possible-effectively to use means for sealing and cooling at the tube base plate. In order to bring about the corresponding changes in the how rate of the air to be heated, the battles u in the recuperator illustrated in Figure 1, in which the air is conducted in a zigzag path past the tubes b, are provided at diflierent distances apart, that is to say, the plate 0 on the entry side of the air is closer to the upper tube base plate m than the distance between the remaining plates u. The same applies to the plate p relatively to'the tube base plate e. The air thus flows into the recuperator at n at a relatively high speed, so that the temperatures at the upper tube base plate In areconsiderably reduced. On the further path, the cross-section of the air movement, and thus the velocity of the air, are such that the coetficient of heat transmission of the air exceeds the coefficient of heat transmission of the hot gas flowing in the tubes. At the lower end, the air leaves the recuperator at q the air velocity being likewise increased due to the shorter distance between the plate p and the base plate e,

effective operation.

.According to the invention furthermore, packings in -the.form of the packing rings illustrated in Figure 2 of in position by a ring 11 with corresponding screw thread arrangement consisting of plate 16 which exerts a'force on rings 11 and 13 as nut 14 is tightened down onthe shank of bolt 15 so that a sealing effect in the downward .1

direction is obtained. The pressure exerted on the resilient ring 13 is also exerted against the tube walls as a result of the radial deformation of said ring.

Instead of by screws, the pressure may also be exerted by means of wedges or the like. It is thus possible to use one or several packing rings, and when forming the packing rings, all the shapes known in engine pistons may be used.

In addition to the resiliency or elasticity, the advantages of the packing consist in that the packing is removable and no welding whatever is required. This is of great importance, because it has been found that due to the welding of packing members to the upper tube base plate, which is, cooler, stresses are set up, which lead to slight twistings and thus to leakages.

In spite of the increased cooling eifect of the air, it is possible that the packing rings at the cooler end of the tubes may still be endangered. The temperature of the waste gas even when leaving the tube b at the upper end is still very high and might cause the packing rings to be loosened or the packing rings may even be oxidised, as the air flows directly around them.

According to the invention furthermore, therefore, a sleeve 17 is suspended in and secured in the tubes. A dead air space 18 which serves as an insulation and thus protects the packing rings is formed between the walls of this sleeve and those of the tube.

The resilient packing hereinbefore described is only an example and may be modified. When the temperatures are very high, it may, for example, be advantageous to move the packing further down into the air-filled space so as to obtain even more efiicient cooling by the flowing air.

Finally, the entire bearing may be covered by a loose insulating mass 19 which is inserted in order to protect in addition the bolts 15 and the nut 14, as well as the whole bearing of the packing rings from the efiects of the hot waste gases.

Thus, when at the upper tube base plate a bearing is provided, which resists even the highest temperatures, it is necessary with these extreme temperatures to take special precautions for the lower tube base plate. It has been attempted to protect the lower tube base plate e from overheating by placing a second highly alloyed metal plate at a distance in front of it, and packing the space thus formed with insulating material. The protective plate had thus only to support the insulating mass so that there was no objection to making it of a sheet metal which was highly alloyed and was accordingly inclined to become brittle, seeing that any cracks caused.

by brittleness are, harmless. However, it was found that in spite of the perfectly movable suspension, the plate could not be made to withstand the strain. Due to the unavoidable differences in temperature, deformations of the plate occurred which led to the destruction of the plate or otherwise rendered it useless.

According to the invention furthermore, the downwardly extending tubes are widened downwardly as from approximately. 40 mm. above the tube base plate. The

space between the tubes is lined with an insulating mass f, which can withstand high temperatures. For example, insulating concrete or the ramming mass used for lining cupola furnaces can conveniently be used.

This type of insulation s and f for the plates has a high degree of durability in spite of the difference between the coefficients of expansion of the metal plates and of: the insulating mass. .Due to the tubes being :tubes should be elastic in the axial direction. plest way of providing this elasticity is by means of self- 4 widened to form trumpet mouths, the advantage is obtained that there is practically no eddying of the entering gases, resulting at the hot end in a desirable reduction of the coeflicient of heat transmission of the entering heating gases.

There is, however, still a danger of the widened parts of the tubes containing the insulating mass and around which no air flows, becoming prematurely scaled. According to the invention, therefore, to avoid this happening, the insulating layer 1 is made so thin that the tube ends are cooled by dissipation of heat to the plate e which is kept cool. For this purpose, it is essential that the layer should not exceed a thickness of approximately 30 to 40 millimetres.

When the preheating of air exceeds a determined value, approximately 500 C., it will not be possible, in spite of the measures taken as hereinbefore described, to cool the base plate e sufiiciently as it must always assume a higher temperature than the air flowing around it. According to the invention, in these cases, that is to say, when the air is pre-heated to above 550 C., the tube base plate is divided into two separate plates 2 and g, which are connected together by welded-in sleeves r. The tubes b extend through these sleeves and are welded to the lower end of the sleeve. In this manner, the steel construction formed is rigid.

According to the invention furthermore, cold air is blown in through the space 12 between the two plates which, as illustrated in the drawings, enters at i and escapes at j. The volume and velocity of the air in this space are so determined that a pre-heating of the air to approximately 300 C. is obtained. The air thus preheated then emerges from the double base plates at I and is conducted at one position, at 1:, into the main air 'stream, at which position the temperature of the main air stream is likewise approximately 300 C. in this manner, a cooling of the base plate is obtained, which is so eifective that it is possible without difiiculty to obtain in the space of the main recuperator disposed above it a pre-heatiug of the air of for example 800 C. Due to the cooling air thus pre-heated to a low temperature being returned to the main air stream, the cooling of the plate in the thermodynamic sense is effected almost without loss.

In order to prevent the normally not very eifectivc cooling of the highly pre-heated air flowing over the plate g from becoming excessive, the plate g may be provided at the top with a thin insulating layer s.

The invention may be modified in various directions.

For example, not only is it useful in steel rccuperators, in which the air is moved to and fro, but the air can also be conducted helically about one or several tubes through which the gases flow.

It is important that thepacking at the top end of the The simtightening packing rings. However, it is also possible to use rings, which consist of several parts which partly overlap, or follow each other closely, and which are made to fit the tubes by a radially efiective elastic means, such for example as by means of a helical spring which is disposed around the packing ring.

The problem of providing steel recuperators even for the highest waste gas temperatures in furnaces is according to the invention, solved for the first time, so that fields of application are opened up which hitherto have been considered unattainable.

We claim:

1. A steel recuperator comprising a cylindrical casing, means in said casing for defining a sealed heat exchanging space, said means including an upper partition member and a lower partition member, tubes mounted with their respective ends in said members and extending through said space, an inlet for gas to be heated leading into the upper part of said space, and an outlet for the heated gas from the lower part of said space, means mounted in the lower part of said casing for heating gas to pass through said tubes, baflie means mounted in said casing for increasing the velocity of the gas to be heated in the passage through the cylinder, the lower partition member comprising two horizontally disposed plates extending completely across said cylinder and a space extending between said plates from one side to the other of said cylinder for the circulation of cooling air between the plates, and inlet and outlet pipes connected respectively to opposite sides of said space, said outlet pipe passing the air for entry to said heat exchanging space to constitute a portion of the gas to be heated thereby.

2. A steel recuperator comprising a cylindrical casing, means in said casing for defining a sealed heat exchanging space, said means including an upper partition member and a lower partition member, tubes mounted with their ends in said members and extending through said space, an inlet for gas to be heated leading into the upper part of said space, and an outlet for the heated gas from the lower part of said space, means mounted in the lower part of said casing for heating gas to pass through said tubes, baffie means mounted in said casing for increasing the velocity of the gas to be heated in the passage through the cylinder, said upper partition member comprising an I upper insulating member mounted in parallel spaced relation with respect to a lower plate, both the member and the plate being perforated at corresponding positions for the passage of the respective ends of the tubes, the lower plate being formed with annular recesses at positions around the tubes to receive packing rings and packing ring retaining means adapted to exert radial and axial pressure on said rings, and screw means for maintaining the upper insulating member and the lower plate in pressure engagement to sandwich between them the packing rings and packing ring retaining means, the lower partition member comprising two horizontally disposed plates extending completely across said cylinder and a space extending between said plates from one side to the other of said cylinder for the circulation of cooling air between the plates, and inlet and outlet pipes connected respectively to opposite sides of said space, said outlet pipe passing the air for entry to said heat exchanging space to constitute a portion of the gas to be heated thereby.

3. A recuperator according to claim 1, in which insulating packing is provided around the lower ends of each tube at positions between theltwo horizontally disposed plates.

4. A recuperator according to claim 2, including a sleeve suspended in the upper end of each of said tubes to leave a dead air space between the sleeve and the tube so as to serve as an insulation and protection for the packing rings.

References Cited in the file of this patent UNITED STATES PATENTS 370,865 Hague et a1 Oct. 4, 1887 500,701 Abbott et al. July 4, 1893 1,683,804 Reed Sept. 11, 1928 1,894,956 Kerr 1 Jan. 24, 1933 1,894,957 Lucke et a1. Jan. 24, 1933 2,196,683 Pickstone Apr. 9, 1940 2,411,097 Kopp Nov. 12, 1946 2,615,688 Brumbaugh- Oct. 28, 1952 FOREIGN PATENTS 608,433 France Apr. 23, 1926 

